Support the WIN64/EFI64 calling convention on all X86_64 platforms

Add a new calling convention FFI_EFI64, alias FFI_WIN64, on all X86_64
platforms.  This allows libffi compiled on a 64-bit x86 platform to call
EFI functions.

Compile in ffiw64.c and win64.S on all X86_64 platforms.  When compiled
for a platform other than X86_WIN64, ffiw64.c suffixes its functions
with _efi64, to avoid conflict with the platform's actual
implementations of those functions.

configure.host

@@ -246,7 +246,7 @@ case "${TARGET}" in
 	SOURCES="ffi.c sysv.S"
 	;;
   X86_64)
-	SOURCES="ffi64.c unix64.S"
+	SOURCES="ffi64.c unix64.S ffiw64.c win64.S"
 	;;
   X86_WIN64)
 	SOURCES="ffiw64.c win64.S"